1. Field of the Invention
This invention relates to a multi-layered composite gasket and method of making a gasket.
2. Related Art
Gaskets are used in many applications to perfect a fluid-tight seal between two mating members. The gasket is typically clamped between the mating members and held there in compression. In the case of internal combustion engines, many gaskets are used in various capacities. As one example, the head gasket is one which is positioned between the cylinder block and cylinder head of an engine to prevent the leakage of combustion gases from the combustion chambers during use of the engine. However, the head gasket in this application not only seals the combustion chambers, but also various coolant and lubrication flow passages that extend between the cylinder block and head.
An example of a prior art head gasket is illustrated in FIG. 1, with four large, generally circular openings representing the periphery of four side-by-side combustion chambers. A plurality of smaller openings are shown dispersed about a gasket for conducting coolant, lubrication and accommodating bolts used to establish clamping pressure. It will be readily apparent that a gasket, such as the exemplary head gasket, must be capable of maintaining a fluid-tight seal for various fluid mediums, including high temperature/high pressure gases, water or ethylene glycol-based coolants, lubricating oils and the like all operating at different temperatures, different pressures, and having different chemical compositions.
Accordingly, designing a gasket to function satisfactorily over an extended duty range can be quite difficult. The prior art has developed numerous gasket styles and compositions for these purposes, some of which are better suited to certain applications. One such gasket construction known in the prior art consists of a multi-layered composite gasket like that shown in FIGS. 2 and 3, and which correspond, generally, to the gasket described in U.S. Pat. No. 5,468,003 to Staab et al., issued Nov. 21, 1995, the disclosure of which is incorporated here by reference. This patent describes a cylinder head gasket used predominantly in high performance diesel engine applications. The gasket is composed of five layers, including a steel center layer to which is bonded a flanking perforated steel layer on each side. The perforated steel layers have outwardly extending tangs which establish a mechanically interlocking engagement with outer graphite layers. These five layers—center, two perforated, and two graphite—comprise the body of the gasket assembly. Combustion openings are rimmed with fire rings dressed with a thin sheet metal fire ring holder which directly engages the multi-layered composite gasket body.
While the prior art gasket construction depicted in FIGS. 2 and 3 is known to be moderately effective in certain applications, there are aspects of the construction that could be improved to gain better performance. In particular, the solid core center layer is affixed directly to the two facing perforated steel layers by an adhesive compound for the purpose of resisting combustion pressures. The outer graphite facing layers are mechanically bonded to the perforated metal layers to provide a fluid seal against the clamping faces of the cylinder head and block (shown in simplified fashion in FIG. 2). The rigid adhesive glue joint provides a sealing function between the perforated and center metal layers.
In use, a gasket in an engine is subjected to varying loads from the forces of combustion and heating cycles which impart sheer stresses to the gasket as the head and block expand and contract at different rates or otherwise move relative to one another due to dynamic forces. Sheer loads are thus created in the body of the gasket, which can weaken and possibly break the rigid glue layers between the perforated metal layers and the central metal core. It has been observed that sheer loads resisted within the body of the gasket can weaken and possibly break the rigid glue layers and thereby present a possible leak path for fluids through the gasket body. If the glue layers do not break under sheer loading, then the sheer loads are borne almost entirely by the graphite layers, putting these layers under undue sheer stresses which can, over time, compromise their ability to maintain a good seal against the mating members. Thus, prior art gasket constructions like that depicted in FIGS. 2 and 3 suffer from shortened service life because of failure modes resulting from undue sheer stresses.
Accordingly, there is a need in the art for an improved multi-layer composite gasket construction of the type to be clamped between mating members for perfecting a fluid-tight seal therebetween.